PROJECT SUMMARY/ABSTRACT Coxiella burnetii is the causative agent of human Q fever, a zoonotic disease that can cause a debilitating, flu- like illness in acute cases, or a life-threatening endocarditis in chronically infected patients. Q fever patients present with few distinguishing clinical features, and chronic disease requires a minimum of 18 months of antibiotic treatment, highlighting the need for new therapeutics. An obligate intracellular pathogen, Coxiella survives inside a vacuole with characteristics of a functional phagolysosome, including active proteases and phosphatases and moderately acidic pH, a physicochemical parameter to which C. burnetii is exquisitely adapted. C. burnetii is a strict moderate acidophile capable of efficient nutrient transport and catabolism and replication only within a narrow pH range under both host cell-free and intracellular conditions. The objective of this application is to identify host and pathogen factors that maintain both CCV and bacterial cytoplasmic pH. Aim 1 will test the hypothesis that C. burnetii manipulates host lysosomal biogenesis by blocking activation of the transcription factor TFEB. Aim 2 will test the hypothesis that C. burnetii controls bacterial cytoplasmic pH via carbonic anhydrase-dependent generation of HCO3- and active uptake of K+. Understanding the molecular mechanisms behind C. burnetii survival within the moderately acidic environment of the CCV will allow identification of potential therapeutic targets.